meowgorithm/defaults

Enforce default values on structs in Go

Defaults

Enforce default values on struct fields.

type User struct {
	Name     string  `default:"Goku"`
	Power    float64 `default:"9000.01"`
}

var u User

err := defaults.Apply(&u)
if err != nil {
	log.Fatal("Uh oh: %v", err)
}

fmt.Print(u.Name)  // Goku
fmt.Print(u.Power) // 9000.01

Defaults are only applied to fields at their zero value.

type Config struct {
	Host  *string `default:"0.0.0.0"`
	Port  *int    `default:"8000"`
}

var cfg Config
json.Unmarshal([]byte(`{Host: "charm.sh"}`), &cfg)

if err := defaults.Apply(&cfg); err != nil {
	log.Fatal("Rats: %v", err)
}

fmt.Print(cfg.Host) // charm.sh
fmt.Print(cfg.Port) // 8000

Works well with JSON, Yaml, and so on.

type Config struct {
    Host  string `json:"host" default:"0.0.0.0"`
    Port  int    `json:"port" default:"8000"`
    Debug bool   `json:"debug" default:"true"`
}

Supported Types

The following types are supported:

• string
• bool
• int
• int8
• int16
• int32 (and rune, with some caveats)
• int64
• uint
• uint8
• uint16
• uint32
• uint64
• float32
• float64
• []byte/[]uint8

…as well as pointers to those types.

Embedded Structs

Embedded structs are supported. The following will parse as expected:

type GroceryList struct {
	Fruit struct {
		Bananas int `default:"8"`
		Pears   int `default:"12"`
	}
	Vegetables *struct {
		Artichokes    int `default:"4"`
		SweetPotatoes int `default:"16"`
	}
}

Embedded structs do not need a default tag in order to be parsed. Embedded structs that are nil will be initialized with their zero value so they can be parsed accoringly.

Runes and Int32s

In Go rune is just an alias for int32. This presents some ambiguity when parsing default values. For example, should "1" be parsed as a literal 1 or as a unicode '1' (which has the int32 value of 49)?

Because of this ambiguity we recommend avoiding setting defaults on runes. That said, this package defaults to parsing int32 as integers. Failing that, it tries to parse them as a rune.

// This works as expected...
type Cool struct {
	Fave32BitInteger int32 `default:"12"`
	FaveChar         rune  `default:"a"`
}

// ...but these will not.
type UhOh struct {
	FaveChar rune `default:"3"`  // this is a unicode ETX or ctrl+c
	FaveChar rune `default:"97"` // this is a unicode `a`
}

Licnse

MIT